While I've always strived for symetry and proportion, as well as mass and strength, I've found that, for me at least, it's almost inevitable for some muscles to lead the pack in development while others stubbornly lag behind. This leads me to wonder how many others here are in the same boat.

In my case my calves are easily my most well developed muscle group (really two muscle groups), followed by my triceps. Meanwhile, my deltoids and biceps are my worst. They're not terrible, ...just not on par with the rest of my body. I credit this mostly to genetics.

Best: Pectorals, both in hardness and in strength (if not purely in size)
Worst: Biceps, mostly due to some recent neglect (basically I traded biceps for all leg muscles... a trade I would make again given the same circumstances)